DNA: The Molecule of Life

1
DNA The Molecule of Life

• Molecular Genetics

2
DNA and RNA

• Genes are segments of DNA on a chromosome that
  code for specific traits
• DNA nucleic acid called deoxyribonucleic acid
  that contains the instructions necessary for a
  cell to build proteins from amino acids.
• RNA ribonucleic acid plays a role in gene
  expression and protein synthesis

3
Isolating the Material of Heredity

• 1869 Friedrich Miescher isolated a weakly acidic
  phosphorus-containing substance from the nuclei
  of white blood cells
• Called it nucleic acid
• Early 1900s Pheobus Levene isolated two types
  of nucleic acid
• Called them ribose nucleic acid (RNA) and
  deoxyribose nucleic acid (DNA)
• Soon after, Thomas Hunt Morgan provided the first
  experimental evidence that genes are located on
  chromosomes
• Working with fruit flies

4
Isolating the Material of Heredity

• In 1928 Fredrick Griffith designed an experiment
  to study the bacteria that were responsible for a
  pneumonia epidemic in London
• He discovered that the dead pathogenic bacteria
  passed on their disease-causing properties to
  live, non-pathogenic bacteria
• He called this the transforming principle
• Griffith died during world war II but several
  scientists built on his work

5
http//www.juliantrubin.com/bigten/dnaexperiments.
html
6
Isolating the Material of Heredity

• In 1944, Oswald Avery, Colin MacLeod, and Maclyn
  McCarty discovered
• When they treated heat-killed pathogenic bacteria
  with a protein-destroying enzyme, transformation
  still occurred
• When they treated heat-killed pathogenic bacteria
  with a DNA-destroying enzyme, transformation did
  not occur
• The results provided evidence that DNA has a role
  in transformation

7
http//courses.cm.utexas.edu/emarcotte/ch339k/fall
2005/Lecture-Ch8-1.html
8
Isolating the Material of Heredity

• In 1952, Alfred Hershey and Martha Chase used
  radioactive labeling to show that genes are made
  of DNA
• They used a virus that contains a protein coat
  surrounding a length of DNA
• This virus attaches to a bacteria cell and
  injects genetic information into the cell
• The infected cell produces new viruses and bursts
  which releases the new viruses to infect other
  cells

9
Isolating the Material of Heredity

• Hershey and Chase created two batches of the
  virus
• In one they labeled the protein coat with
  radioactive sulfur
• In the other, they labeled the DNA with
  radioactive phosphorus
• They found that the radioactive phosphorus was
  found in the bacterial cells
• They concluded that DNA must direct the cell to
  produce new viruses
• animation

10
http//www.accessexcellence.org/RC/VL/GG/hershey.h
tml
11
Structure of DNA

• After isolating DNA and RNA, Levene determined
  that both molecules are made up of nucleotides
  (in long chains)
• Nucleotide is composed of
• 5-carbon sugar (deoxyribose in DNA, ribose in
  RNA)
• Phosphate
• Nitrogen base (4 different types)
• The 4 nitrogen bases belong to two chemical
  groups called purines and prymidines
• Purines Adenine (A) and Guanine (G)
• Prymidines Thymine (T) and Cytosine (C)

12
(No Transcript)
13
Structure of DNA

• In the late 1940s Erwin Chargaff found that
  nucleotides are not present in equal amounts in
  DNA and RNA
• Nucleotides are present in varying proportions
• He found that the number of adenine in DNA is
  equal to the number of thymine in a sample
• The amount of cytosine is approximately equal to
  the amount of guanine
• This constant relationship is called Chargaffs
  rule
• Video on Chargaffs rule

14
Structure of DNA

• In the early 1950s, a British scientist
  Rosaslind Franklin used x-ray photography to
  analyze the structure of DNA
• DNA has a helical structure with two regularly
  repeating patterns
• Nitrogenous bases are located on the inside of
  the structure, and the sugar-phosphate backbone
  is located on the outside (near the watery
  nucleus)
• Many argue that Franklin should have shared in
  the Nobel Prize for discovering the structure of
  DNA, but she died before it was handed out
• NOVA program on photo 51.

15
Structure of DNA

• In 1953, James Watson and Francis Crick were the
  first to produce a structural model for DNA
• Watson and Cricks model of DNA closely resembles
  a twisted ladder
• Deoxyribose sugar and phosphate molecules make up
  backbone (handrails of the ladder)
• Paired nitrogen bases held together by weak
  hydrogen bonds make up the rungs (steps) of the
  ladder
• The ladder twists to form a double helix
• From Franklins images, Watson and Crick knew the
  distance between the sugar-phosphate handrails
  remained constant

16
Structure of DNA

• The two strands that make up DNA are not
  identical, they are complementary to eachother
• This is due to the complementary base pairs of
  A-T and C-G
• The two strands are also antiparallel
• The phosphate bridges run in opposite directions
  in the two strands
• Each end of a double stranded DNA molecule
  contains the 5 end of one strand and the 3 end
  of the complementary strand

17
http//www.synapses.co.uk/genetics/tsg19.html
18
(No Transcript)
19

• In a segment of DNA, the number of purines equals
  the number of pyrimidines this is because of the
  base pairing rule
• RULE nitrogen bases always pair in complementary
  pairs
• Adenine Thymine
• Guanine Cytosine

20

• Ex) if 15 of the bases were thymine, what
  percentage would be
• a) adenine
• b) guanine
• c) cytosine

21
Example

• Determine the complementary strand of DNA
• A T G C A G C
• I I I I I I I

22
Ribonucleic Acid (RNA) compared to DNA

• The sugar component of RNA is ribose
• RNA does not have the nucleotide thymine (T), in
  its place is uracil (U)
• RNA remains single stranded
• There are several types of RNA
• mRNA, rRNA, tRNA
• DNA, RNA animation

23
DNA Replication (Synthesis)

• Replication DNA has the ability to replicate
  (or duplicate) itself.
• This is why one cell is able to divide into two
  cells and each cell has identical genetic
  information
• Replication takes place during S phase of
  interphase

24
DNA Replication (Synthesis)

• Replication is semi-conservative
• Each new molecule of DNA contains one strand of
  the original complementary DNA and one new parent
  strand
• Each new strand conserves half of the original
  molecule
• Meselson-Stahl Experiment
• Replication takes place at several locations
  along the DNA molecule simultaneously
• The steps are described in sequence to better
  understand the concept
• BioFlix Replication

25

1. Replication starts at a specific nucleotide
   sequence called the replication origin
2. During replication, weak hydrogen bonds that hold
   complementary nitrogen bases together are broken
   (This causes the two edges to unzip) with a
   special group of enzymes called helicases (gyrase
   breaks the hydrogen bonds)
3. This creates two y-shaped areas (replication
   forks) at the end of the unwound area, the
   unwound area is called a replication bubble
4. The parent (original) strands are conserved while
   two new strands created from nucleotides are
   formed with them (they act as a template)

26

• Free floating nucleotides (from diet) are
  attached to the exposed nitrogen bases according
  to the base pair rule with an enzyme called DNA
  polymerase
• This process is called elongation
• DNA polymerase attaches new nucleotides to the
  free 3 end of a preexisting chain of nucleotides
• Elongation can only take place in a 5 to 3
  direction
• This means that replication occurs in opposite
  directions along each strand of the parent DNA
• One strand is replicated continuously, it is
  called the leading strand
• The other strand is replicated in short segments,
  it is called the lagging strand
• The short segments are called Okazaki fragments
• These fragments are spliced together by an enzyme
  called DNA ligase

27

• Since DNA polymerase cannot synthesize new
  fragments of DNA, and RNA primer serves as a
  starting point for the elongation of each new DNA
  strand
• An enzyme called primase is required to construct
  a primer
• When finished the strand, DNA polymerase removes
  the primer by eliminating the nucleotides in a 5
  to 3 direction
• Hydrogen bonds form between the nitrogen bases
• Special proofreading enzymes (DNA polymerase)
  check the new strand of DNA for mistakes. Errors
  are removed by cutting the mistake out and using
  an endonuclease and replacing it with the correct
  nitrogen base

28

• As soon as the newly formed strands are complete,
  they rewind automatically into the helix
  structure
• Replication continues until the new strands are
  complete and the two DNA molecules separate from
  eachother
• This is called termination
• This replication produces two strands of DNA from
  one where each strand is composed of half-old
  and half-new

http//www.bio.davidson.edu/Courses/Molbio/MolStud
ents/spring2005/Durnbaugh/yfp.html
Replication Fork Adding Nucleotides Replication
Animation Replication Review
29
http//www.biosci.ohio-state.edu/mgonzalez/Micro5
21/04.html
http//distancelearning.ksi.edu/demo/bio378/lectur
e.htm
30
Genetic Engineering and Recombinant DNA

• Human Genome Project - animation
• Genetic Engineering refers to the alteration of
  an organisms genome (complete set of genes) by
  selectively removing, adding, or modifying DNA
• Recombinant DNA the process of cutting out DNA
  from one genome and placing the DNA into another
  genome resulting in a transgenic organism

31

• Examples of transgenic organisms
• Genetically modified bacteria for use in medicine
  and bioremediation (environmental clean-up)
• Transgenic plants to improve crop yield and
  resistance to environmental effects
• Cloned animals (livestock) and organs for human
  use

32
Recombinant DNA How do they do it?

• Use restriction enzymes (endonucleases) to cut
  strands of DNA within their interior (at specific
  sequences)
• animation
• Then ligase (enzyme that fuses segments of DNA)
  is used as a biological glue

33
Production of human insulin

• Gene in the human genome that codes for insulin
  is cut out using restriction enzymes
• The plasmid of an E-coli bacteria is cut using
  restriction enzymes so that the gene for insulin
  can be inserted using a ligase
• Bacteria can read the DNA and produce insulin for
  us to later harvest and use

34

• Another example is the insertion of the gene that
  codes for growth hormone into animals so that
  they grow faster
• Note Biotechnology refers to the use of
  organisms or biological products for commercial
  and/or industrial processes
• - video

35

• What are the Issues?
• Costs/where money is spent
• Motivation for the product
• Biological characteristics of the product
• Heath effects
• Environmental effects
• Freedom of Information/Privacy
• Who Owns the technology/patents
• Issues Animation

36
Gel Electrophoresis

• Technique used to separate DNA fragments by size
  for the purpose of identification in paternal or
  criminal suits (animation)
• Sample of DNA is cut using restriction enzymes
  from hair, blood, skin, etc. This produces a
  number of DNA segments of different lengths.
• The different pieces of DNA (referred to as
  restriction-fragment-length-polymorphisms or
  RFLP) are tagged with a radioactive isotope

37

• 3) Using an agarose gel that contains holes or
  wells along one side, the samples of DNA are
  inserted into the wells. A known sample is
  loaded with it as a comparison

38

• 4) Electric current is run through the gel,
  causing the movement of the negatively charged
  DNA fragments. The shortest strands move the
  farthest (lowest weight) and the longer strands
  (heavier) will not move as far.

39

• 5) This causes separation of the DNA into bands.
  The gel is left to set
• 6) When combined with staining or X-ray film, the
  patterns are used to determine the presence or
  absence of particular DNA or proteins

40
(No Transcript)
41
DNA Fingerprinting

• Developed in 1985 used to identify whether or
  not a sample of DNA comes from a specific person
• People have similar DNA, however every human
  (with the exception of identical twins, triplets,
  etc.) have some unique noncoding segments of DNA
  called introns exons are segments of DNA that
  actually code for proteins

42

• Sample of DNA is placed through gel
  electrophoresis as well as samples from
  individuals who are suspected as owners of the
  sample
• Because of introns, each individual will have a
  different number of sites where the restriction
  enzyme will cut
• This results in a unique number and length of
  fragments which produces a unique banding pattern
  (fingerprint) when x-rayed
• Fingerprints are used to identify criminals,
  paternity or kinship
• Animation

43

• Lane A DNA from crime scene cut with Enzyme 1
• Lane B DNA from crime scene cut with Enzyme 2
• Lane C DNA from Suspect 1 cut with Enzyme 1
• Lane D DNA from Suspect 1 cut with Enzyme 2
• Lane E DNA from Suspect 2 cut with Enzyme 1
• Lane F DNA from Suspect 2 cut with Enzyme 2

44
Protein Synthesis

• Genetic code is determined by the arrangement of
  nitrogen bases within the strands of DNA
• Each gene codes for the production of a specific
  protein
• DNA RNA protein

translation
transcription
45
Proteins

• Proteins are composed of 20 different amino acids
  that are strung together in endless combinations
• Compose cell membranes, cell organelles, muscle
  filaments, hair, hair color, enzymes (regulate
  speed of chemical reactions in cells), antibodies
  (disease-control agents), hormones

46
Genetic Code

• It takes the code of 3 nucleotides (a codon) to
  code for one amino acid
• Humans can make 12 of the 20 amino acids, we must
  consume the other 8 essential amino acids
• Simple protein 8 amino acids
• Complex protein 50 000 amino acids
• Sequencing amino acids is determined by DNA
• Replacement of a single amino acid can change a
  protein

47
Genetic Code

• The genetic code has three important
  characteristics
• The genetic code is redundant (more than one
  codon can code for the same amino acid)
• The genetic code is continuous (reads as a series
  of three letter codons without spaces,
  punctuation or overlap)
• The genetic code is nearly universal (almost all
  organisms use the same code this is good for
  genetic engineering and biotechnology)

48
Role of DNA in protein synthesis

• DNA is in nucleus, but protein synthesis occurs
  on the ribosomes in the cytoplasm
• Carrier molecule (mRNA messenger RNA) is
  responsible for reading the information from the
  DNA (transcription) and carry it to the ribosomal
  RNA (rRNA) in the cytoplasm where it will be
  translated into an amino acid sequence by
  transfer RNA (tRNA)

49
RNA (Ribonucleic Acid)

• Different from DNA in that
• Its single stranded
• It contains the sugar ribose
• It is located throughout the cell (DNA is only in
  the nucleus with some also in the mitochondria)
• It contains the base uracil (U) instead of
  thymine (T)
• There are three types mRNA, rRNA, tRNA
• Its shorter (no introns)

50
Transcription

• Protein synthesis begins with transcription (RNA
  synthesis) of DNA
• DNA never leaves the nucleus (protected)

51
Steps of Transcription

• DNA molecule unzips (like in replication),
  however, RNA nucleotides are now added to the
  necessary areas (exons) by RNA Polymerases
• For each gene, only one strand of the DNA is
  transcribed, this is called the sense strand.
  The other is called the anti-sense strand.
• As double helix uncoils, nucleotides from the
  mRNA find the appropriate pair by using single
  DNA strand as a template (5 to 3 direction)
• Uracil binds to exposed adenine bases and
  cytosine binds to exposed guanine bases
• mRNA joined and fused in a long chain
• mRNA move away from DNA and the DNA strands
  rejoin again
• mRNA leave the nucleus in search of the ribosomes

52
http//fig.cox.miami.edu/cmallery/150/gene/mol_ge
n.htm
53

• _____________________________DNA
• I I I I I I I I
• A G C T T A T C
• U C G A A U A G
• I I I I I I I I
• _____________________________RNA

54

• mRNA reads code from DNA
• RNA codes for amino acids
• Some codes in mRNA are not for amino acids but
  are terminators and initiators
• Terminators end protein synthesis (stop codon)
• Initiators turning protein synthesis on (start
  codon). Also called promoter site, starts RNA
  transcription
• Transcription Animation

55
http//www.coolschool.ca/lor/BI12/unit6/U06L01.htm
56
Example

• Original DNA sequence
• AAT GCC AGT GGT TCG CAC AAA
• Write the complementary DNA sequence
• Write the mRNA sequence
• How many amino acids are there?
• What is the amino acid sequence?

57

• Do the same for the following DNA sequence
• TAC CAC GTG GAC TGA GGA CTC CTC ATC ATA

58
Translation

• Translation is the next stage of protein
  synthesis
• Involves translating codons found on the mRNA
  into a chain of amino acids (to form a protein)
• Transfer RNA, tRNA is made up of a single strand
  of RNA that folds into a clover-leaf shape
• One lobe contains the anticodon, three
  nucleotides that are complementary to the mRNA
  codon
• At the opposite end is a binding site for the
  amino acid that corresponds to the codon
• Ribosomal RNA, rRNA is a linear strand of RNA
  that remains associated with the ribosomes

59
More on tRNA

• The exposed bases are called the anticodon
• Each kind of tRNA molecule has a specific
  anticodon
• Ex) Glutamate carried by a tRNA molecule that
  carries either the CUU or the CUC anticodon
  (opposite to mRNA codons)
• Ex) Valine carried by a tRNA molecule that has
  the CAA anticodon

60
(No Transcript)
61
Steps of translation

1. Translation is activated when an mRNA molecule
   binds to an active ribosome complex in such a way
   that two codons are exposed
2. The first tRNA molecule carrying the amino acid
   methionine, base pairs with the start codon on
   the mRNA, (AUG)
3. Another tRNA molecule arrives at the codon next
   to the first tRNA, and the first tRNA passes its
   amino acid on to the second tRNA
4. Enzymes catalyze the formation of a peptide bond
   between the two amino acids
5. The ribosome moves along the mRNA strand one
   codon at a time
6. The first tRNA molecule detaches from the mRNA
   and picks up another amino acid as another tRNA
   attaches to the mRNA.

62

• NOTE The process begins with the presence of an
  initiator (start codon) AUG and ends with the
  presence of a stop (terminator codon) UAA, UGA,
  or UAG on the mRNA.
• Remember that the sequence of amino acids was
  originally derived from the message carried by
  mRNA from the nucleus (DNA)
• Translation animation 1
• Translation animation 2 Translation 3
• Protein Synthesis Process

63
http//users.rcn.com/jkimball.ma.ultranet/BiologyP
ages/T/Translation.html
64
http//www.scq.ubc.ca/?p263
65
Example

• Write a tRNA and amino acid sequence for the
  following DNA sequence
• TAC CAC TGA GGA CTC CTC CAT CAT

66
Mutations

• A mutation is a permanent change in the DNA
  sequence caused by mutagens (mutagenic agents)
• Mutations
• are inheritable
• Arise from mistakes in DNA replication when one
  nitrogen base is substituted for another
• Creates a new genetic code that gives new
  instructions to make amino acids (causes a
  different protein to be made)

67
Mutagenic agents

• Physical
• Pushing or tugging chromosomes
• Chemical
• Carcinogens, mustard gas, poor nutrition
• Medications
• Some antibiotics
• Radiation
• X-ray, ultraviolet radiation, cosmic rays
• Replication mistakes
• Natural mistakes occur during mitosis or meiosis
• Nutritional
• Lacking certain nucleotides in diet means you are
  unable to provide the proper free nucleotide base
  and this causes a mismatch
• Biological
• Most viruses use genetic material of chromosomes
  to reproduce. They join existing DNA to cause
  permanent damage

68

• Mutation Animation
• If a mutation is present in the gametes, it will
  be passed on to further generations. This is why
  it is particularly dangerous for pregnant women
• Mutations can be grouped under 2 categories
• 1) Chromosomal mutations
• 2) Point mutations

69
Chromosomal mutations

• Large mutations that visibly effect the structure
  or number of chromosomes
• Ex) nondisjunction, fragile-X-syndrome

70
Point Mutations

• Alter a single gene. There are several types
• 1) Base substitution a foreign base replaces
  the normal base in each strand of DNA. This
  could result in one amino acid being different
  animation
• ACGCCA becomes CCGCCA
• Ex) Sickle cell anemia substitution of one
  nitrogen base causes an inability to carry
  sufficient oxygen

71

• 2) Insertion A base is added into the normal DNA
  sequence
• ACGCCA becomes AACGCCA
• 3) Deletion a base is removed from the normal
  DNA sequence
• ACGCCA becomes CGCCA
• Ex) Cystic fibrosis deletion of 7th, 8th, and
  9th nitrogen bases causes an inability to produce
  protein that regulates chloride channels
• animation

72

• NOTE both insertion and deletion result in a
  frame-shift mutation because every amino acid
  after the point of mutation may be affected
• 4) Translocation a sequence of nitrogen bases is
  removed from one area and placed in another
• ABCDEFGHIJ becomes ABFGHIJCDE

73

• 5) Inversion reversal of a sequence of nitrogen
  bases
• ABCDEFGHIJ becomes ABEDCFGHIJ
• 6) Duplication duplicating a set or sequence of
  nitrogen bases twice in one location
• ABCDEFGHIJ becomes ABCABCDEFGHIJ
• 7) Silent mutations no phenotypic effect because
  certain amino acids have more than one code
• GTA (CAU) and GTG (CAC) both code for histidine
• NOTE The body can repair some mutations, but not
  all

74
Oncogenes and Cancer

• In normal cells, protein synthesis is carried out
  by structural genes only when required
• Because protein synthesis is not always required,
  a regulator gene produces a repressor protein
  which switches off protein synthesis and reducing
  the rate of cell division
• P53 animation

75

• Uncontrolled cell division is cancer.
• Cancer is caused by a mutation due to an
  environmental factor or carcinogen
• Mutation could be a base substitution that
  prevents the production of the repressor protein,
  or the movement of a gene from one part of the
  chromosome to another
• If the structural gene is separated from its
  regulator gene, it cannot be turned off (cancer
  forms)
• These genes that cannot be turned off are called
  oncogenes

76

• Most common oncogene ras
• Found in 50 of colon cancers and 30 of lung
  cancers
• Ras makes a protein that acts as an on switch
  for cell division. Once a sufficient number of
  cells are produced, it should shut off
• Cancer-causing oncogene produces a protein that
  blocks the off switch (causes cell division to
  continue at an accelerated rate)

77
Diagnosing Genetic Disorders

• Amniocentesis and Chorionic Villus Sampling can
  take cell samples from a developing fetus or
  embryo
• This sample can be screened for genetic markers
  for certain disorders
• Uses a DNA probe which identifies problem genes

78
Gene Therapy

• Targets genetic causes of diseases rather than
  their symptoms
• A DNA vector carries foreign DNA into target
  cells of the patient
• The vector is usually a virus that has been
  genetically altered to carry a desired gene
• The virus will eventually transfer the new gene
  into the cells genome

79
Gene Therapy

• Somatic Gene Therapy correcting genetic
  disorders in somatic cells
• Mutations can still be passed on to offspring
• Germ-line Therapy correcting the genetic
  information in sperm and egg
• Could have many negative effects on future
  generations
• Currently banned in Canada

80
Ames Test

• Technology to determine quickly, cheaply, and
  accurately if a chemical is mutagenic.
• Any chemical that is mutagenic has potential to
  be carcinogenic
• We must test products for their cancer-causing
  agents

81

• Performed on bacteria that have been mutated so
  they cannot produce histadine on their own (we
  must supply them with it in order for them to
  survive)
• Plate this bacteria on a petri dish with no
  histidine and the chemical being tested (expect
  no growth)
• If bacteria are found, conclusion can be made
  that microbe has been mutated and is now
  producing its own histidine

82

• The more colonies that are found, the higher the
  strength of mutagenic the chemical is which
  indicates it is highly carcinogenic
• Often chemical is added to liver enzymes because
  chemicals are often harmless to an individual
  until they are broken down in the liver into
  toxic metabolites
• NOTE some chemicals can cause cancer in some
  individuals and not in others (because of
  different nitrogen base sequences in each
  individual)

83
Biological Warfare

• Most disease-causing agents can be exploited for
  biological weaponry
• Microbe or toxin produced by microbe may be
  harmful to livestock, grains, bacteria in soil,
  or humans
• Fortunately, few organisms are suited for mass
  destruction

84
Examples

• AIDS not transmitted by air
• Clostridium botulinum deadly in water (not
  air)one kg of toxin in water reservoir kills 50
  000 people (60 of population dies in 24 hrs)

85
Research/Testing Stations

• Britain Porton Down
• USA Camp Detrick in Maryland
• Canada Suffield in Alberta
• Preferred microbe was anthrax bacillus (affects
  cattle and humans).

86
Anthrax

• Deadly spores (rod-shaped bacterium) live long
  periods of time, are highly contagious, and
  resistant to many environmental factors
• We currently have the ability to create the
  superbug through merging genes for rapid
  reproduction and environmental resistancewhat do
  you think would happen then?

87
Mitochondrial DNA

• Mitochondria responsible for cellular
  respiration
• 1960s discovered that mitochondria contains
  its own DNA (mtDNA)
• They have an amount of control over what they do
  (not completely controlled by nuclear DNA)

88
Endosymbiotic Hypothesis

• Mitochondria once were free living bacteria that
  were engulfed by other cells.
• The two cells developed mutualistic relationship
  (mitochondria had protection and food, engulfing
  cell had a source of energy and oxygen)

89
Evidence to support theory

• MtDNA resembles the loops of DNA found in
  bacteria and viruses
• The mtDNA is tiny compared to nuclear DNA
• Mitochondria divide and replicate independently
  of the cell itself
• the same theory is used to explain how
  photosynthetic cells gained chloroplasts
• NOTE The mtDNA in our bodies is maternal because
  sperms mitochodria are lost when their tail
  falls off.